Add Rsbe::Client::Collections

Highlights:
* add ```Rsbe::Client::Collections``` specs and implementation
* add ```#get_children``` method to ```Rsbe::Client::Base``` to allow access to nested resources
* add ```Rsbe::Client::Partner#collections``` that returns an array of ```Collection``` objects for the ```Partner```
* add ```Rsbe::Client::MethodNotImplementedError```
* light housekeeping